Method for manufacturing thin-film lithium microbatteries

ABSTRACT

A process for making thin-film batteries including the steps of cleaning a glass or silicon substrate having an amorphous oxide layer several microns thick; defining with a mask the layer shape when depositing cobalt as an adhesion layer and platinum as a current collector; using the same mask as the preceding step to sputter a layer of LiCoO 2  on the structure while rocking it back and forth; heating the substrate to 300° C. for 30 minutes; sputtering with a new mask that defines the necessary electrolyte area; evaporating lithium metal anodes using an appropriate shadow mask; and, packaging the cell in a dry-room environment by applying a continuous bead of epoxy around the active cell areas and resting a glass slide over the top thereof.

This is a divisional application of U.S. patent application Ser. No.09/779,595 filed on Feb. 8, 2001, now U.S. Pat. No. 6,558,836.

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work undera NASA contract, and is subject to the provisions of Public Law 96-517(35 USC 202) in which the contractor has elected not to retain title.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to thin-film batteries compatible withintegrated circuit manufacturing and in particular to the method formaking the same.

2. Description of Related Art

There is an ever-expanding field where solid-state thin-film batteriesmay be used, including critical space exploration systems. Examplesinclude battery-backed CMOS memory, MEMS switches, and micro sensors. Inmany of these applications, the ability to integrate a thin film batterydirectly onto a silicon integrated circuit chip next to the device to bepowered would reduce overall device mass and volume as well as open upthe possibility of distributing power at the chip level. The ability todeposit high capacity films onto flexible polymer substrates is alsoappealing.

The ideal thin-film battery has a capacity approaching theoreticallevels for the materials system selected, minimal capacity loss overthousands of cycles, and materials and process steps that are compatiblewith common silicon-based fabrication methods.

In the recent past, RF magnetron sputtered LiCoO₂ thin films haveemerged as a leading candidate for use as the cathode layer in thin filmsolid state batteries. Films that exhibit a (104) or (101) out-of-planetexture, have grains that are >100 nanometers in diameter, and displaylittle to no lattice strain and are most efficient at lithiumintercalation and electronic conduction. It has been shown elsewherethat an annealing step of typically 700° C. induces the desired filmqualities. Because this heating step is too high for many desirablesubstrates, such as flexible polymer materials or silicon wafers withintegrated CMOS devices, an effort has been undertaken to examine thepossibility of creating LiCoO₂ films with the desired material qualitieswithout using a high temperature process step. To reach this goal, it iscritical to understand the deposition parameters that affect filmcrystallography and composition of LiCoO₂ when sputtered at roomtemperature.

A range of compositions have been observed in sputtered LiCoO₂. TheLi/Co ratio has been reported to be 0.88 [9] or 1.0+/−0.1 when sputteredusing a sputter gas consisting of 100% Argon. If sputter gases compriseAr:O₂ mixes of 1:10 and 1:1, the Li/Co ratio was found to be 0.8+/−0.08and 1.15+/−0.02, respectively. Concurrently, O/Co ratios ranging from2.7 to 2.2 have been reported as well. In all cases, these films werefound to be amorphous (per x-ray diffraction analysis) if deposited atroom temperature and subsequently developed primarily a (003), (101), or(104) out-of-plane texture upon annealing at temperatures in excess of600° C., depending on thickness.

In contrast to these results, preliminary experiments show that it ispossible to deposit nano-crystalline LiCoO₂ films at room temperature.Some of these films have strong (104) out-of-plane textures and arefound to have promising electrochemical properties.

SUMMARY OF THE INVENTION

The highest capacity thin film cathode layers (LiCoO₂) typically requirean annealing step of 700° C. Since this high temperature is notcompatible with silicon device technology or flexible polymersubstrates, the development of a low process temperature (<300° C.)cathode layer is needed. LiCoO₂ thin-films were RF sputter deposited andsubsequently incorporated into thin film batteries. A variety ofdeposition and post-deposition parameters were varied in an effort tooptimize film microstructure and content. Film composition andmicrostructure were examined using a variety of techniques includingx-ray diffraction using synchrotron radiation. It was found that LiCoO₂could be deposited at room temperature in a nano-crystalline state witha strong (104) out of plane texture and a high degree of latticedistortion. By heating these layers to 300° C., the grain size issignificantly increased while lattice distortion is eliminated. Cyclingdata reveals that the heating step increases cell capacity to neartheoretical values (at lower discharge currents) while significantlyimproving both the rate capability and discharge voltage.

Accordingly, it is an object of the present invention to provide a thinfilm battery that has near-theoretical capacities and good dischargecapabilities without exceeding temperatures of approximately 300° C. andare compatible with silicon processing techniques, which was heretoforea major problem.

Another object of the present invention is to provide a method forgrowing films with nano-crystalline grains oriented in the propercrystallographic direction at room temperature.

Yet another object of the present invention is to provide a simplifiedmethod for manufacturing thin-film batteries by heating the films to300° C. in order to create cathode layers that have near state of theart performance.

These and other objects, which will become apparent as the invention isdescribed in detail below, are provided by a process for makingthin-film batteries including the steps of cleaning glass or a siliconsubstrate having an amorphous oxide layer several microns thick;defining with a mask the layer shape when depositing cobalt as anadhesion layer and platinum as a current collector; using the same maskas the preceding step to sputter deposit LiCoO₂ in an argon-oxide gas onthe structure while rocking it back and forth; heating the substrate to300° C. for 30 minutes; sputtering with a new mask that defines thenecessary electrolyte area; evaporating a lithium metal anodes using anappropriate shadow mask; and, packaging the cell in a dry-roomenvironment by applying a continuous bead of epoxy around the activecell areas and resting any thin protective (i.e., insulating) over thetop thereof.

Still other objects, features and advantages of the present inventionwill become readily apparent to those skilled in the art from thefollowing detailed description, wherein it is shown and described onlythe preferred embodiment of the invention, simply by way of illustrationof the best mode contemplated of carrying out the invention. As will berealized, the invention is capable of other and different embodiments,and its several details are capable of modifications in various obviousrespects, all without departing from the invention. Accordingly, thedrawings and description are to be regarded as illustrative in nature,and not as restrictive, and what is intended to be protected by LettersPatent is set forth in the appended claims. The present invention willbecome apparent when taken in conjunction with the following descriptionand attached drawings, wherein like characters indicate like parts, andwhich drawings form a part of this application.

BRIEF DESCRIPTION OF THE DRAWINGS

The general purpose of this invention, as well as a preferred mode ofuse, its objects and advantages will best be understood by reference tothe following detailed description of an illustrative embodiment withreference to the accompanying drawings in which like reference numeralsdesignate like parts throughout the figures thereof, and wherein:

FIG. 1A is a plan view of the beginning steps of the method of thepresent invention.

FIG. 1B is a cross-sectional view of the structure shown in FIG. 1A.

FIG. 2A is a plan view of an intermediate step of the method of thepresent invention.

FIG. 2B is a cross-sectional view of the structure shown in FIG. 2A.

FIG. 3A is a plan view of an intermediate step of the method of thepresent invention.

FIG. 3B is a cross-sectional view of the structure shown in FIG. 3A.

FIG. 4A is a plan view of the penultimate step of the method of thepresent invention.

FIG. 4B is cross-sectional view of the structure shown in FIG. 4A.

FIG. 5 is a plan view of the resulting structure following steps of thepresent invention.

FIGS. 6A and 6B illustrate the results from the synchrotron x-raydiffraction studies of the structure in accordance with the presentinvention.

FIG. 7 illustrates normalized (104) X-scan texture data of the structureof the present invention.

FIG. 8 illustrates the TEM images of 80 nm thick LiCoO₂ films sputteredonto holey carbon grids in accordance with the present invention.

FIG. 9 shows the electrochemical performance of thin film batteriescomprising as-deposited 200 nm thick LiCoO₂ cathodes.

FIG. 10A shows a plot of cell potential vs. capacity for a LiCoO₂ thinfilm battery with a 0.190 μm thick cathode.

FIG. 10B contains a plot showing the cycle life of a typicalheat-treated LiCoO₂ cell.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is provided to enable any person skilled inthe art to make and use the invention and sets forth the best modescontemplated by the inventors of carrying out their invention. Variousmodifications, however, will remain readily apparent to those skilled inthe art, since the general principles of the present invention have beendefined herein specifically to provide low process temperaturenano-crystalline LiCoO₂ thin-film cathodes.

Referring now to the drawings and FIG. 1A in particular, a plan view ofa beginning step of the process of the present invention is shown. Thispart of the process is for creating a large area thin-film cathode layerof LiCoO₂ that has favorable crystallographic texturing and grain sizefor use in a thin-film solid state battery system. The process beginswith a clean glass substrate 10 having an amorphous oxide layer 11several microns thick. With the use of a shadow mask (not shown) thebeginning elements 12 and 13 of the battery are formed by depositing 100nm of cobalt as an adhesion layer 14 and 300 nm of platinum as a currentcollector layer 15. Deposition conditions are 10 mTorr of argon, at 300W of rf power, with a substrate to target distance of 70 mm. Thesubstrate is kept stationary during growth of layers 14 and 15. Thecross-sectional view of FIG. 1B shows the same structural elements withthe same reference numerals. The beginning elements 12 and 13 areisolated from each other as shown in FIG. 1A.

Referring now to FIG. 2A, a plan view of an intermediate step of themethod of the present invention is shown. The same shadow mask (notshown) used in forming the structure shown in FIGS. 1A and 1B is used inthis step as well. Hence, the plan view of FIG. 2A is the same as theplan view of FIG. 1A. However, as can be seen in FIG. 2B, an additionallayer 16 of LiCoO₂ is deposited to form the cathode. The sputter gas isa 3:1 Ar:O₂ mix with a total pressure of 10 mTorr. The substrate totarget distance is 70 mm. However, a key difference is depositing thislayer 16 is that the substrate is rocked back and forth underneath thetarget at an amplitude of 5 cm. This motion serves to create an ovulararea (approximately 3×5 cm²) on the substrate within which the desired(104) out of plane texture is found. The shadow mask for the cathodelayers must be situated within this area to insure proper layerperformance. The film thickness can range from 5 to 5000 nm (at least),where the thicker the film corresponds to batteries with a highercapacity.

The substrate is removed from the vacuum sputter chamber (not shown) andis kept in a controlled dry (, 1% humidity) environment. X-raydiffraction is performed on these layers to insure that the cathodelayer contains nano-crystalline LiCoO₂ grains oriented with their (104)planes parallel to the plane of the substrate. The substrate is thenplaced in an oven and heated to 300° C. and held at this temperature for0.5 hours. The oven is located in the dry environment to insure thatmoisture contact with the cathode layer is minimized. The resulting filmlayers should have a fully relaxed LiCoO₂ hexagonal crystal structure,have some grains that approach 30 nm in diameter, and have a strong(104) out of plane texture.

Referring now to FIG. 3A, a plan view of the deposition of the LiPONsolid electrolyte layer 18 is shown. The substrate and newly addedlayers is placed back into the sputter chamber with a new shadow mask(not shown) that defines the electrolyte area, thereby forming the layer18. It is important that the sputter chamber be very clean beforestarting this step. The substrate to target distance is 90 mm this time,and the substrate holder is a massive metallic object that is kept inthermal contact with the substrate, which helps to cool the substrateduring deposition. This precautionary step helps to overcome anynon-uniform heating problems. Approximately 1 micron of LiPON is thendeposited onto the cathode layer 16 of elements 12 and 13. The LiPON issputtered from a pressed and sintered LIPO₄3″ sputter target in pure N₂environment. A target power of 100 watts is used. The substrate is againrocked back and forth during growth of the layer 18 as well. Therefore,through a new shadow mask, an electrolyte layer 18 is formed bysputtering a thin film solid state electrolyte material onto the cathodelayer 16 while rocking the substrate 10 in an oscillatory fashion.

It has been discovered empirically that the rocking action eliminateslocalized heating and averages the film composition and thickness overthe important areas deposited, thereby greatly reducing lateralvariations in the deposited LiPON layer 18. The electrolyte or LiPONlayer 18 is grown to a thickness of 1 to 1.5 μm. The electrolyte lithiumphosphorus oxynitride (LiPON) is a recently discovered material that hasgood lithium ion conductivity and, more importantly, is stable incontact with metallic lithium at high voltages.

Referring now to FIG. 4A, a plan view of the resulting structure isshown after a lithium metal anodes 20 is evaporated using an appropriateshadow mask (not shown). In particular, the structure is removed fromthe deposition chamber, while still warm, and transferred to anevaporation chamber. Using the anode shadow mask, less than 2 microns oflithium are evaporated onto the layer 18, thereby forming the anodelayer 20. FIG. 4B shows this same structure in cross-section where thelithium metal anode 20 contacts the top portion of the electrolyte layer18 above a portion of the beginning element 12 with an isolated portionof the cathode layer 16 above beginning element 13, as shown in FIGS.1A-4B. This portion above beginning element 13 functions as part of theanode circuit.

Following the above, the structure is sealed in a dry environment byplacing a glass cover over it and attaching with epoxy. Morespecifically, the structure is placed in a dry environment (<1%humidity) to cool after the preceding step of forming the anode layer20. The active anode/electrolyte area is surrounded with a bead of 5minute epoxy 21. Care is to be taken here to assure that the leads fromthe anode and current collector are not fully enclosed by the epoxybead. Next, a thin insulating protective layer 22 (such as glass) isplaced over the active area such that a hermetic seal is created usingthe epoxy. The assembly is then allowed to dry. As can be seen in theplan: view of FIG. 5, anode lead 23 and a cathode lead 24 are exposedfor connection to external circuitry. The result is the thin filmbattery in accordance with the present invention, which is shown in FIG.5.

The final step is to connect the battery to a circuit. In order to dothis, part of the LiCO₂ layer 16 is removed in order to form a goodohmic contact. The part removed is that part exposed outside of thepackaging, which is removed by using HCl acid soaked into a wiper. Theexposed part is then cleaned with methanol. After this, a bead of solderis melted onto the exposed part of the platinum layer 15 and anelectrical lead is welded to the layer 15 by means of the solder. Asshown in FIGS. 4B and 5, a portion of the cathode layer 16 not coveredby the electrolyte layer 18 is removed with HCl acid to expose thecurrent collecting layer 15. The exposed portions of the currentcollecting layer 15 form an anode 23 and a cathode 24 comprising the twoterminals of the thin-film battery. After exposing and cleaning theanode 23, and the cathode 24, an ohmic contact may be formed between thetwo terminals of the thin-film battery and a circuit.

The highest capacity thin-film cathode layers (LiCoO₂) typically requirean annealing step of 700° C. Since this high temperature is notcompatible with silicon device technology or flexible polymersubstrates, the development of a low process temperature (e.g., <300°C.) cathode layer is required. LiCoO₂ thin-films are RF-sputterdeposited and subsequently incorporated into thin film batteries. Avariety of deposition and post-deposition parameters are varied in aneffort to optimize film microstructure and content. Film composition andmicrostructure are examined using a variety of techniques includingx-ray diffraction using synchrotron radiation. It was found that LiCoO₂could be deposited at room temperature in a nano-crystalline state witha strong (104) out of plane texture and a high degree of latticedistortion. By heating these layers to 300° C., the grain size issignificantly increased while lattice distortion is eliminated. Cyclingdata reveals that the heating step increases cell capacity to neartheoretical values (at lower discharge currents) while significantlyimproving both the rate capability and discharge voltage.

The highest capacity per unit volume for a thin film battery, >65μAh/cm² μm, has been reported from cells fabricated using a LiCoO₂cathode layer in conjunction with a LiPON electrolyte and lithium anode(theoretical capacity approximately 69 μAh/cm² μm). These cellsdisplayed excellent cycle life (over 10,000 cycles), discharge ratecapability (mA range), and discharge voltages (>3.9 V). However, it wasalso reported that a 700° C. anneal of the cathode layer was required;the use of this high temperature precludes the use of many substratesand is not compatible with silicon device processing techniques.Nano-crystalline LiMn₂O₄ cathode can be prepared at temperatures lessthan 300° C., but they display diminished capacities and cannot beeffectively discharged at currents greater than 50 μAh/cm².

In an attempt to create the highest possible capacity thin-film batterysystems, LiCoO₃ is selected as a cathode material. Though there havebeen numerous reports that LiCoO₂ forms a completely amorphous film whenRF sputter deposited at room temperature, preliminary work indicatesthat the desirable polycrystalline or nano-crystalline microstructurescould be created at room temperature.

Three-inch lithium cobalt oxide (LiCoO₂) sputter targets are prepared bycold pressing and sintering standard powders. The sintering temperatureis set to 800-900° C. for 1 hour, and target thicknesses are 4-6 mm. Thetargets are subsequently bonded to aluminum backing plates using moltenindium or silver epoxy.

The growth chamber is a custom-designed turbopumped 3-position RF planarmagnetron sputter system configured in the “sputter down” geometry. Arotating substrate platen allows for the growth of multiple films ondifferent substrates, with substrate to target distance of 7 cm. Themaximum base pressure before deposition is 3×10⁻⁶ Torr, though mostfilms are grown at base pressures lower than 1×10⁻⁶ Torr. Typically, theLiCoO₂ targets are pre-sputtered for 30 minutes before opening theshutter. The cathode films are sputtered in Ar:O₂ ratios ranging from3:1 to 1:1 with a total gas flow rate of 55 sccm and pressure of 10mTorr. Nominal target power is set to 100 W, giving a power density of2.2 W/cm². To eliminate lateral compositional variations in the film,the substrate platen was programmed to oscillate underneath the sputtertarget through a range of 12° at a frequency of 0.3 Hz, a motion thatswept the substrate back and forth. This motion produces an area ofapproximately 4×5 cm on the substrate with uniform color, thickness, andcomposition. LiCoO₂ deposition rate varies linearly depending on theratio of argon to O₂ in the sputter gas; films deposited in 100% argonhad a deposition rate of 210+/−5 nm/hr, while a 1:1 mix of argon and O₂resulted in a growth rate of 175+/−10 nm/hr.

The cathode films are deposited onto cleaved test-grade silicon wafers(with native amorphous oxide intact), and C tape for compositional andstructural analysis, while glass with molybdenum or platinum currentcollectors are used as a substrate for cell creation. Thicknesses aredetermined using an Alpha-Step 100 profilometer. Test cell cathode areasare defined using aluminum masks that define an area of 1.4 cm². Aftergrowth, the films are transferred to a dry room (<1% humidity air) wherethey are subjected to a heating step of 300° C. for 1 hour. Completethin film batteries are then fabricated using these cathodes and using asputtered LiPON electrolyte layer and an evaporated Lithium anode. TheLithium anode is evaporated in a Denton thermal evaporator situatedinside the dry room. Cathode layer composition may be determined using acombination of Rutherford back scattering (RBS) and inductively coupledplasma mass spectroscopy (ICP-MS).

Film crystalline structure is examined using x-ray diffraction conductedat the Stanford Synchrotron Radiation Laboratory (SSRL). The use of asynchrotron x-ray source is justified because these thin (typically <0.4μm), low atomic weight films yield poor counting statistics usinglaboratory-based x-ray systems, particularly when using the θ-2θgeometry. A silicon (111) monochrometer is used to select the wavelengthof the x-ray beam, 0.124 nm (10 keV), in focused mode. To quantitativelyexamine texture, the out of plane substrate tilt, X was scanned while θand 2θ were set to satisfy a particular Bragg condition. This type ofscan is essentially a cross section of a pole and will be referredherein as a texture plot. The films are kept under a constant heliumenvironment during data collection to avoid moisture contamination.

To examine microstructure and grain size, plan-view transmissionelectron microscopy may be performed on films that are deposited ontoholey carbon grids. All samples are kept in an air-tight, dryenvironment until immediately before insertion into the TEM.

The electrochemical characteristics of the batteries are examined bycycling them using an Arbin BT-2042 battery testing instrument, whilecyclic voltammetry measurements are performed using a PAR 273potentiostat. Typically, the cells are charged to 4.25 V using a maximumcurrent of 50 μA, and discharged to 2.5 V at currents ranging from 2 μAto 1 mA. During the charging cycle, the cells are held at 4.25 V whilethe charging current decreased to less than 2 μA.

Compositional Analysis

RBS and ICP-MS analysis show that LiCoO₂ film composition depends uponthe growth history of the target and composition of the sputter gas. Thefilms grown for the studies described herein are from targets with 5-30hours of sputtering and have Li/Co ratios of 1.1+/−0.1 with a O/Co ratioof 2.2+/−0.1. These results are consistent with values cited in theliterature and indicate that it is likely that a small amount of Li₂O isformed during sputtering.

X-ray Diffraction

Results from the synchrotron x-ray diffraction studies are shown inFIGS. 6A and 6B, wherein FIG. 6A illustrates a diffraction patterncollected from a LiCoO₂ powder standard; that is, collected at SSRL,powder x-ray diffraction pattern of LiCoO₂, λ=1.24A. FIG. 6B illustratesdiffraction from 150 nm thick LiCoO₂ RF sputter films, before and afterthe 300° C. heating step.

Using a beam energy of 10 keV (λ=1.24 A), 5 peaks can be observed in the2θ range of 10° to 40°, including the (003), (101), and (104), which arethe 3 strongest reflections reported in the literature for sputteredhexagonal LiCoO₂ films. FIG. 6B contains diffraction patterns collectedfrom a 150 nm thick LiCoO₂ film sputtered in a 1:1 Ar/O₂ sputter gas.Silicon substrate and air scattering have been subtracted from thesepatterns, such that diffraction from only the films is shown. A broad(104) peak is observed for the as-deposited film, showing that thislayer is composed of (104) out of plane textured nano-crystallinegrains. The effect of heating this film to 300° C. is illustrated inFIG. 6A, where the (104) peak becomes sharper and moves to the predictedequilibrium 2θ position, at 36.05°.

FIG. 7 illustrates normalized (104) X-scan texture data. The (104) θ-2θdiffraction condition is satisfied while the out-of-plane orientation,X, is scanned. The intensity at any X is proportional to the number ofLiCoO₂ unit cells that are oriented X degrees from the substrate surfacenormal.

Grain diameters are calculated using the Scherrer formula, and it wasfound that, as deposited, the grains are 4+/−0.5 nm in diameter, and theheating step increased the average grain diameter to 8+/−1 nm. Similarfilms grown to thicknesses up to 1.4 μm all have a strong (104) out ofplane. It is interesting to note that if the substrate is kept static(i.e., is not in constant motion underneath the target) during growth, astrong (003) texture developed in some film areas.

The 1.1° peak shifts observed as a result of the heating stepcorresponds to a 2.6% change in the out-of-plane d₍₁₀₄₎ latticeparameter. The large value of this change makes it unlikely that this isa simple stress/strain effect. The lattice parameter change observedwould require residual stresses orders of magnitude greater than thoseobserved in thin films.

FIG. 7 shows X scan texture plots of the (104) reflection of LiCoO₂before and after the heating step. These data have been normalized toaccount for the dependence of diffraction volume with respect to X. Theintensity of these normalized plots at any X is proportional to thenumber of unit cells that are in grains that have their (104) planeinclined X° from the substrate plane. The full width at half maximum(FWHM) of a Gaussian curve fit to the data is used as a means toquantify these results. It can be seen that the FWHM decreases from15.4°+/−0.1°1 to 14.3°+/−0.1° as a result of the heating step. Thoughthis change is statistically significant, it is not large and showsthat, while the heating step doubles the average grain size, the degreeof texture is not strongly increased.

The TEM images of 80 nm thick LiCoO₂ films sputtered onto holey carbongrids are shown in FIG. 8, wherein it is shown that a TEM analysis of100 nm thick LiCoO₂ films sputtered onto a holey carbon grid. Brightfield images are shown for a film before and after heating. Significantgrain growth as a result of the heating step may be observed.

The bright field images show that the microstructure is porous andcontains well-defined grains that grow as a result of the heating step.As deposited, all the grains are similar in size, whereas after heating,some of the grains have grown to well over 10 nm in diameter whereasother grains show no change at all. The x-ray determination of grainsize gives only an average value, so these TEM results are valuable inthat they show that heating the films to 300° C. encourages some grainsto more than triple their initial size while other grains display noincrease at all.

Electrochemical Cycling

FIG. 9 shows the electrochemical performance of thin film batteriescomprising as-deposited 200 nm thick LiCoO₂ cathodes. A voltage plateauis found to exist at approximately 2.8 V, while a discharge rate of even50 μA severely limited the cell capacity, indicating that there issignificant electronic resistance in these cells. The cycle life ofthese batteries is found to be limited to 200-300 cycles before losing ¾of original capacity. FIG. 9 also illustrates the cell discharge datafrom a thin film solid state battery created using a LiCoO₂ cathodedeposited at room temperature with no heating step. Discharge currentsrange from 2 to 50 μA/cm².

FIG. 10A shows a plot of cell potential vs. capacity for a LiCoO₂ thinfilm battery with a 0.190 μm thick cathode that was heated to 300° C.for 1 hour. FIG. 10A shows the cell discharge data from a thin filmsolid state battery created using a LiCoO₂ cathode after the 300° C.heating step. Discharge currents range from 2 μA/cm² to 1 mA/cm². FIG.10B shows the charge and discharge capacity versus cycle for a similarcell.

The capacity using a 2 μA/cm² discharge current is 66 μAh/cm² μm (wherethe μm in these units refers to cathode thickness). The dischargevoltage has a value of approximately 3.9 V, however there is noperfectly flat voltage plateau.

FIG. 10B contains a plot showing the cycle life of a typicalheat-treated LiCoO₂ cell. After an initial loss of approximately 20% ofthe original capacity over the first hundred cycles, the capacity levelsout at approximately 46 μAh/cm² μm and maintains at this level for >2000cycles. The average percent capacity loss per cycle equilibrates at−0.003%/Cycle.

The results shown in FIGS. 6-8 illustrate that polycrystalline LiCoO₂films can be produced without any substrate heating during or aftergrowth. This result contradicts four different previously publishedreports that state RF sputtered LiCoO₂ is amorphous as deposited at roomtemperature. There are two possible reasons for this difference: [1]Previous studies did not use synchrotron radiation and so were not ableto detect scattering from thin LiCoO₂ in the θ-2θ geometry for filmsthis thin; or, [2] The combination of growth conditions needed to createthese textured nano-cystalline films are not uncovered in other researchefforts. It is well known that films sputtered from targets of identicalcomposition can differ dramatically microstructurally depending on thesputter system used, lending credence to possibility [2] above.

The as-deposited films display a strong degree of (104) out-of-planetexturing. It has been reported elsewhere that, upon annealing, mostLiCoO₂ films thinner than 1 μm have at least 10-20% of their grainsoriented with the (003) plane parallel to the substrate (thicker filmsare found to develop either (101) or (104)). In fact, an (003) out ofplane texture is observed in certain areas in the films grown in thechamber used for our study if the substrate was held static duringdeposition. This difference is attributed to composition andenvironmental gradients inherent in the magnetron sputter environment.

Upon heating to 300° C. for an hour, three trends are observed for thefilms: [1] the average grain size is found to roughly double. It isfurther established using TEM analysis that some grains grow to over 3times their initial size, while others do not change at all. [2] Thedegree of texturing is found to increase only slightly: from an X-scanFWHM of 15.4° to 14.3°. [3] There is a large change in lattice parameterspacing, as the (104) d-spacing is found to decrease out of the plane ofthe film by approximately 3% while the in plane spacings expand byapproximately 4%. The heating step therefore creates a film containingsome significantly larger LiCoO₂ crystalline grains that were free ofany significant lattice distortion. Electrochemically, it is likely thatthese qualities combine to create films that are more efficientconductors of both ions and electrons.

The analysis of the electrochemical performance of the batteries builtin accordance with this invention indicates that the 300° C. heatingstep significantly increases cell capacity and discharge voltage. Theaverage discharge voltage rose to approximately 3.8 V from approximately2.9 V and the rate capability is enhanced such that increasing thedischarge rate 3 orders of magnitude (from 2 μA to 1 mA) results in a50% loss in overall capacity for the depth of discharge used. This is alarge improvement over the 75% capacity loss observed when the dischargerate is raised to 50 μA from 2 μA in the batteries created without aheating step.

The cycle life for the cells created using heat-treated cathodes isfound to exceed 6000 cycles. During the first 100 cycles, the capacitydecreases approximately 25% and is essentially constant for allsubsequent cycles. This initial decrease in capacity was not observed inall cases, and the cause of this irreversible capacity loss is currentlyunder investigation.

The viability of creating high-capacity, high discharge rate thin-filmbatteries at processing temperatures that do not exceed 300° C. has beenshown. It is established, in contrast to previous findings, thatnano-crystalline highly (104) out-of-plane textured thin rhombohedralfilms of LiCoO₂ can be RF sputter deposited at room temperature and thatthe materials and electrochemical characteristics are enhanced for theselayers upon heating to 300° C. Materials analysis shows that the heatingstep enables a fraction of the grains to quadruple in size while fullyrelaxing any lattice strain. Though the rate capability of these cellsis not as good as that of cells with 700° C. annealed cathodes, the newlow-temperature cells are still able to sustain a 1 mA current load.

While the invention has been particularly shown and described withreference to a preferred embodiment, it will be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention.

Those skilled in the art will appreciate that various adaptations andmodifications of the just-described preferred embodiments can beconfigured without departing from the scope and spirit of the invention.Therefore, it is to be understood that within the scope of the appendedclaims, the invention may be practiced other than as specificallydescribed herein.

What is claimed is:
 1. A method for manufacturing a thin-film batterycomprising the steps of: a. through a first mask, depositing a firstmetal as an adhesion layer on a substrate; b. through said first mask,depositing a second metal as a current collector over said adhesionlayer; c. through said first mask, sputtering a cathode layer in anargon-oxide gas on top of said current collector while rocking saidsubstrate in an oscillatory fashion; d. heating said substrate in a dryenvironment of less than 1% humidity to an elevated temperature for afixed period of time; e. through a second mask, sputtering a thin filmsolid state electrolyte material onto said cathode layer while rockingsaid substrate in an oscillatory fashion, thereby forming an electrolytelayer; f. evaporating an anode layer over said electrolyte layer; and g.packaging and sealing the resulting structure.
 2. The method as in claim1 wherein said first metal is cobalt.
 3. The method as in claim 1wherein said second metal is platinum.
 4. The method as in claim 1wherein said cathode layer is a thin film solid state material.
 5. Themethod as in claim 1 wherein said cathode layer is formed of LiCoO₂. 6.The method as in claim 1 wherein said electrolyte layer is formed ofLiPON.
 7. The method as in claim 1 wherein said step of sealing includesapplying a bead of epoxy around said thin-film battery in a dryenvironment and placing a plate thereover.
 8. The method as in claim 7wherein said plate is glass.
 9. The method as in claim 1 wherein saidsubstrate is silicon having an insulating layer thereover.
 10. Themethod as in claim 1 wherein said substrate is glass.
 11. The method asin claim 1 wherein step d thereof said elevated temperature is 300° C.and said fixed time is 30 minutes.
 12. The method as in claim 1 whereinstep c thereof said step of rocking includes rocking the substrate backand forth beneath the target with an amplitude of 5 centimeters.
 13. Themethod as in claim 1 where step c thereof further includes the ratio ofargon to oxygen gas is in the ration of 3 to
 1. 14. The method as inclaim 1 where step c thereof further includes the step of holding saidsubstrate 70 millimeters away from the target.
 15. The method as inclaim 1 further including after step c, said substrate is maintained ina dry environment of less than 1% humidity while performing an X-raydiffraction on said deposited layers to insure that said currentcollector layer contains nano-crystalline LiCoO₂ grains oriented withtheir (104) planes parallel to the plane of said substrate.
 16. Themethod as in claim 1 wherein step e thereof said electrolyte layer isdeposited in a sputter chamber in the presence of pure N₂ holding saidsubstrate 90 millimeters from the target using 100 Watts of power. 17.The method as in claim 1 wherein step e thereof said electrolyte isdeposited to a thickness of 1 to 1.5 microns.
 18. The method as in claim1 wherein step e thereof further includes rocking said substrate backand forth beneath the target with an amplitude of 5 centimeters.
 19. Themethod as in claim 1 further including removing with HCl acid a portionof the cathode layer not covered by the electrolyte layer to expose aportion of the current collector layer and cleaning the exposed portionof the current collector layer for ohmic contact with circuitryemploying said thin-film battery.
 20. A method for manufacturing athin-film battery comprising the steps of: a through a first mask,depositing cobalt as an adhesion layer on a substrate; b. through saidfirst mask, depositing platinum as a current collector layer over saidadhesion layer; c. through said first mask, sputtering LiCoO₂ as acathode layer in a 3:1 argon-oxide gas on top of said current collectorlayer while rocking said substrate under the target in an oscillatoryfashion; d. heating said substrate in a dry environment of less than 1%humidity to 300° C. for thirty minutes; e through a second mask,sputtering an electrolyte material of LiPON onto said cathode layerwhile rocking said substrate in an oscillatory fashion, thereby formingan electrolyte layer; f. evaporating a lithium metal anodes using anappropriate shadow mask; and, g. packaging and sealing the resultingstructure.
 21. A method for manufacturing a thin-film battery comprisingthe steps of: a. through a first mask, depositing a first metal as anadhesion layer on a substrate, the first metal being deposited in afirst beginning element region and a second beginning element region,the first beginning element region and the second beginning elementregion being isolated from each other; b. through the first mask,depositing a second metal as a current collector over the adhesionlayer, the second metal forming a continuation of the first beginningelement and the second beginning element; c. through the first mask,sputtering a cathode layer in an argon-oxide gas on top of the currentcollector while rocking the substrate in an oscillatory fashion, thecathode layer forming a continuation of the first beginning element andthe second beginning element; d. heating the resulting structure in adry environment of less than 1% humidity to an elevated temperature fora fixed period of time; e. through a second mask, sputtering a thin filmsolid state electrolyte material onto the cathode layer above a portionof the first beginning element while rocking the substrate in anoscillatory fashion, thereby forming an electrolyte layer; f. through athird mask, evaporating an anode layer over a portion of the electrolytelayer above the first beginning element and a portion of the cathodelayer above the second beginning element; and g. packaging and sealingan active area of the structure so that a portion of the first beginningelement not covered by the electrolyte layer and a portion of the secondbeginning element not covered by the anode layer are not enclosed by thepackaging and sealing.
 22. The method as in claim 21, wherein thepackaging and sealing step includes the steps of: applying an epoxy beadaround the active area of the structure; and covering the active area ofthe structure with an insulating protective layer so that the epoxy beadand the insulating protective layer make a hermetic seal.
 23. The methodas in claim 21 further including steps of: h. exposing a portion of thecurrent collector layer covered by the cathode layer not enclosed by thepackaging and sealing; and i. cleaning the exposed portion of thecurrent collector layer.
 24. The method as in claim 23 wherein theexposing step includes removing with HCl acid a portion of the cathodelayer not enclosed by the packaging and sealing to expose a portion ofthe current collector layer.
 25. The method as in claim 23 wherein thecleaning step includes the step of: applying methanol to the exposedcurrent collector layer.
 26. The method as in claim 23 further includingstep of: j. melting a bead of solder onto the exposed part of thecurrent collector layer so that an electrical lead is welded to theexposed current collector layer for making ohmic contact with externalcircuitry.